Thermal Hydrocarbon Chemistry - American Chemical Society

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11 Production of Coke and Other Pyrolysis Products From Acetylene, Butadiene, and Benzene in Various Tubular Reactors LYLE F. ALBRIGHT and YU-HONG CAROL YU Downloaded by TUFTS UNIV on December 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0183.ch011

1

School of Chemical Engineering, Purdue University, West Lafayette, IN 47907

Thermal reactions of acetylene, butadiene, and benzene result in the production of coke, liquid products, and various gaseous products at temperatures varying from 450° to 800°C. The relative ratios of these products and the conversions of the feed hydrocarbon were significantly affected in many cases by the materials of construction and by the past history of the tubular reactor used. Higher conversions of acetylene and benzene occurred in the Incoloy 800 reactor than in either the aluminized Incoloy 800 or the Vycor glass reactor. Butadiene conversions were similar in all reactors. The coke that formed on Incoloy 800 from acetylene catalyzed additional coke formation. Methods are suggested for decreasing the rates of coke production in commercial pyrolysis furnaces.

'"phis project is a continuation of the investigations of Tsai and Albright (I) and of Brown and Albright (2), who earlier studied surface reactions that occur during the pyrolysis of hydrocarbons. Such pyrolyses are used for commercial production of ethylene, other olefins, diolefins, and, to some extent, aromatics. Several important reactions occur on the inner surfaces of the high-alloy steel tubes used for pyrolyses. These surface reactions occur simultaneously and, to some extent, consecutively along with the gas-phase reactions that produce the desired products of Current address: Amoco Chemical Co., Naperville, IL 60540.

1

0-8412-0468-3 / 79/ 33-183-193$05.00 / 0 © 1979 American Chemical Society

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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pyrolysis. The surface reactions are, for the most part, undesired and lead to production of significant amounts of coke, carbon oxides, hydrogen, and even methane. Surface reastions that occur along with coking and decoking including oxidation, reduction, sulfiding, and desulfiding of the inner surfaces of the high-alloy, stainless-steel reactors. Brown and Albright (2) systematically investigated surface reactions when ethylene, ethane, propylene, and propane were pyrolyzed in Incoloy 800, stainless steel 304, and Vycor glass tubular reactors. The importance of surface reactions and of the amounts of coke produced often were significantly affected by the feed hydrocarbon, construction material of the reactor tube, oxidation and sulfiding of the metal reactors, and temperature. More coke was generally produced from ethylene or propylene than from ethane or propane. This finding is consistent with the general conclusion that the more unsaturated hydrocarbons are better coke producers. Stainless steel 304 reactors resulted in more coke than did Incoloy 800 reactors and much more than did Vycor glass reactors (2, 3). For the two metal reactors used, increased concentrations of metal oxides on the inner surface of the reactors often promoted coke formation. Metal oxides are produced when the metal surfaces react either with oxygen or steam at high temperatures. However, metal sulfides that were formed by reactions with hydrogen sulfide or sulfur-containing hydrocarbons, in general, act to suppress coke formation. Coke often contains metal granules, and coking and corrosion of the inner surfaces of the tubular reactors are at least sometimes related, being part of the same phenomena. In general, higher temperatures resulted in increased amounts of coke formation. Acetylene, butadiene, and benzene are thought to be important coke precursors during pyrolysis reactions used to produce light olefins ( especially ethylene) and during dehydrochlorination of 1,2-dichloroethane for production of vinyl chloride. Surface reactions involving these precursors were investigated in the present study. Also, an alonized ( or aluminized ) Incoloy 800 reactor was compared with a regular ( or unalonized ) Incoloy 800 reactor relative to both the production of coke and to surface reactions in general. The alonized reactor that was used frequently resulted in fewer surface reactions. Experimental Details The pyrolysis reactors were similar to those used earlier (1,2); they were 1.1 to 1.26-cm i.d. tubes that were heated in an electrical resistance furnace over a length of about 48 cm. The materials of construction in the four reactors used in this investigation were as follows: Incoloy 800, stainless steel 304, Vycor glass, and alonized Incoloy 800. The latter reactor was prepared by Alon Processing, Inc. of Tarentum, Pennsylvania.

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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The inner surface of an Incoloy 800 tube had been exposed to aluminum vapors at high temperatures; aluminum diffused into the surface, resulting in an aluminized surface. The temperatures reported in this investigation were those near the center point of the reactor that was positioned in the furnace. Maximum temperatures occurred at the center point. Temperatures near either end of the heated section of the tube were less than the maximum temperature by about 40°-90°C, depending to a considerable extent on the maximum temperature used. Temperatures at the center point were controlled at any desired value in the range from about 400° to 800°C. Temperatures in the lower portion of this range were tested since surface reactions at such temperatures are often significant even though gas-phase reactions are not. Hence the technique used provided valuable information relative to the role and importance of surface reactions. Lighter hydrocarbons such as acetylene and 1,3-butadiene were metered at atmospheric pressure to the tubular reactors at a flowrate of about 30 mL/min. Benzene was introduced to the reactor in a mixture containing benzene and helium; helium was bubbled through liquid benzene maintained at approximately 25°C to produce a mixture containing about 12% benzene by volume. The flowrates of the inlet feed streams were such that the residence times of the hydrocarbons in the heated section of the tubular reactors varied from about 25 to 30 sec. The variations of residence times were caused primarily by the differences in the temperature levels used in the reactor and by the variations in the conversions. The product gas stream from the tubular reactor was passed in each run through a glass condenser immersed in an ice bath. Part of the product stream condensed. Both the liquid product and the noncondensed gases were analyzed by using a Tracor dual-column, temperature-programmed gas chromatograph. The columns were constructed from about 180 cm of 6.35-mm stainless steel tubing that was packed with Porapak Q, 80/100 mesh. The chromatograph separated components from hydrogen to at least C hydrocarbons. Material balance calculations were made by using the inlet and exit streams from the tubular reactors to approximate the amounts of coke and other products that collected on or that coated the inner surfaces of the reactor. The amounts of coke or other heavy products left in the reactor also could be determined by a burnout procedure, using oxygen. In this procedure, the amounts of carbon oxides and water in the exit stream were measured at frequent intervals during the burnout. In general, reasonable agreement was noted in the values of coke and other heavy products, as calculated by these two methods. Oxygen burnouts generally resulted in mainly carbon oxides but in little water vapor; the products left in the reactor are hence thought to be primarily carbon. î 2

Acetylene Results Six runs were made using acetylene as the feedstock. Both the kinetics of acetylene conversions and the product composition frequently varied significantly depending on the reactor used, the immediate pre-

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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treatment of the reactor, and the temperature. Table I outlines the material of construction of the reactor, the pretreatment, and range of conditions investigated for each of the six runs. In these runs, significant amounts of gaseous, liquid, and solid (or coke) products were formed. The liquid product contained appreciable amounts of benzene; butadiene, C , other C , and C hydrocarbons also were present. The noncondensed or gaseous products were, in each case, primarily hydrogen and methane. Trace amounts of ethane, propylene, and butadiene also were detected. 5

6

7

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Table I.

Run

Runs with Acetylene

Immediate Pretreatment

Reactor

14

Incoloy 800

15

Incoloy 800

1,3-butadiene runs to form coke on surface 0 at 700°C,190 min H at 700°C, 1120 min 0 at 700°C, 150 min H , at 700°C, 1020 min C H at 25 °C, 30 min 0 at 800°C, 570 min H at 700°C, 1050 min 0 at 700°C, 280 min H at 700°C, 1100 min H S at 700°C,40min 2

Temperature Range Investigated (°C) 350-650 350-550

2

18

Incoloy 800

19 21

Vycor Glass Alonized Incoloy 800

23

Incoloy 800

2

6

2

6

450-500 400-650 350-650

2

2

450-600

2

2

Figure 1 indicates an example of how pretreatment of the Incoloy 800 reactor had a very large effect on the acetylene conversion (or on the kinetics of acetylene decomposition). The coke-coated Incoloy 800 reactor (the coke had been deposited on this reactor when butadiene reacted at 500°-700°C.) used in Run 14 resulted in much lower acetylene conversions in the range of 450° to 550°C compared with the same Incoloy 800 reactor after the coke had been burned off with oxygen and after the reactor had been contacted with hydrogen until nearly all surface oxides were eliminated. Most conversion results for the 11 gas samples collected during Run 15 are shown in Figure 1. Gas Samples 1 through 3 at 350°, 400°, and 450°C, respectively, indicated almost no acetylene conversions. A small amount of carbon dioxide was produced at 450°C, indicating some metal oxides had still been present on the surface after the hydrogen pretreatment. The temperature was then increased to 500°C, and the conversions then increased from 66% to 99% during the first 23 min ( Samples 4 and 5 ). Some carbon oxide production was noted in Sample 4 but none in Sample 5 or in any later samples of the run; presumably

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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Figure 1. Acetylene conversions in coke-coated Incoloy 800 and clean Incoloy 800 reactors. Acetylene feed: (A) Run 14, coke-coated Incoloy 800; (Π) Run 15, clean Incoloy 800. metal oxides on the surface had reacted and had been reduced before Sample 5 was taken. The conversion remained at about 99% for more than one hour as the temperature was maintained at 500°C, as indicated by Sample 7. At 500°C and higher, large amounts of hydrogen were produced. Obviously coke also was formed simultaneously. When the temperature was decreased to 450°C (Sample 11), a conversion of about 11% occurred. An earlier sample (Sample 3) taken at 450°C had indicated little or no conversion. Run 15 was terminated after about 4.7 hr. The Incoloy reactor was pretreated prior to Run 18 similar to that before Run 15. For Run 18, substantial conversions were noted at 450°C, whereas none were detected in Run 15 until a temperature of 500°C was reached. Conversions of 85%-100% were noted in the range 4 7 5 ° 500° C in Run 18. After about 2.33 hr, the inlet pressure of the reactor began to rise, indicating a plug was starting. The run then was termi­ nated. When the reactor was opened, it was found to be almost plugged with coke. This coke was magnetic in nature, and metal particles were visually observed in it. Analysis with a scanning electron microscope equipped with E D A X has confirmed the presence of metals in cokes formed from acetylene on Incoloy 800 surfaces (3). The mechanism that explains how metal granules are incorporated in the coke and how these

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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granules promote the formation of additional coke has been presented by both Lobo et al. (4,5) and Baker et al. (6,7,8). Cokes formed, however, from acetylene on alonized Incoloy 800 surfaces are nonmagnetic and do not contain metals except for small amounts of aluminum (probably as alumina) (3). Cokes on Vycor glass surfaces also contain no metal. An important finding of this investigation was that cokes formed from acetylene on Incoloy 800 surfaces caused what appears to be an ever increasing rate of coke formation, as indicated by the results of Run 15 and especially of Run 18. In other words, this coke resulted in an autoacceleration phenomenon. Yet the coke formed from butadiene (as indicated by the results of Run 14) seemed to deactivate the surface so that a slow and rather steady rate of coke formation occurred. The reason for this difference in the rates of coke formation will be discussed later in this chapter. Prior to Run 23, the reactor was contacted with hydrogen sulfide for about 15 min. Acetylene conversions of Run 23 increased from about 94% at 450°C to almost 98% at 600°C. The conversion results at a given temperature were quite reproducible during this run that totalled almost 3 hr in length. Yields of gaseous and coke products were significantly less than those of Runs 15 and 18 even though acetylene conversions were often similar. Yields of liquid products were much greater in Run 23 than in Runs 15 and 18. Liquid yields for Run 14 were, however, greater than those of Run 23. The results for Run 19 (Vycor glass reactor), Run 21 (alonized Incoloy 800 reactor), and Run 14 (coke-covered Incoloy 800 reactor) were similar to both the kinetics and type of products obtained. Although neither oxygen or hydrogen pretreatments were tried in Vycor glass or alonized Incoloy 800 reactors prior to acetylene pyrolyses, it is thought that such pretreatments would have little or no effect on acetylene reactions. This conclusion is based on such pretreatments prior to pyrolysis with other hydrocarbons in these two reactors. It has been concluded that all increases in acetylene conversions above those of Runs 14, 19, and 21 were in some way caused by surface reactions. Based on this assumption, surface reactions were of major importance in Runs 15, 18, and 23. Butadiene Results 1,3-Butadiene reacted in Incoloy 800, alonized Incoloy 800, and Vycor glass reactors to give quite similar results. Butadiene conversions increased from about 60%-80% at 500°C to about 94%-97% at 700°C. The main products formed were liquids (i.e., products that condensed

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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at 0 ° C ) and were obviously condensation products. Analysis of the liquid products with the gas chromatograph indicated at least 25 peaks. The three peaks that were much larger than the others tentatively have been identified as benzene, toluene, and 4-vinylcyclohexene. Yields of liquid products decreased from about 65%-79% at 500°C to 57%-63% at 700°C. At 500°C, 4-vinylcyclohexene was by far the most important product, but significant amounts of benzene also were present. At 700°C, both benzene and toluene were more important, totaling about 75%-80% of the liquid product. The results of this investigation are consistent with thefindingsof Sakai et al. (9). They had found 4-vinylcyclohexene to be the main product at low conversions of butadiene for temperatures ranging from 550° to 750° C. They also reported mechanisms to explain the production of benzene, cyclohexadiene, and cyclohexene. The gaseous products for the butadiene reactions consisted mainly of hydrogen, methane, ethylene, ethane, and propylene. The first three mentioned were the major gaseous products at 500°C, but about 50% of the gaseous product was methane at 700°C. Yields of gaseous products increased from about 5%-7% at 500°C to 32%-35% at 700°C. The "solid phase" retained in the reactor during butadiene reactions was probably a mixture of tar (or heavy hydrocarbons) and coke. This conclusion is based in part on material balance calculations. These calculations, although not considered highly accurate, suggest that heavy hydrocarbons or tar were formed in appreciable amounts at lower temperatures such as 500°C. Yields of surface-deposited products were about 16%-26% at 500°C but were only 5%-10% at 700°C. At the higher temperatures, calculations indicated the deposits were almost completely coke. Heavy hydrocarbons deposited at lower temperatures probably were decomposed rather completely to coke and hydrogen when the reaction temperature was increased to 700°C during the final stages of the run. Visual observation of the Vycor glass reactor immediately following the butadiene run at 700°C resulted in important information. The reactor was cut to permit inspection of the black deposits thought to be primarily coke. The last two-thirds of the reactor and a short section of the unheated tube that extended beyond the furnace were covered on the inner surface with a smooth layer of coke. This deposit, when viewed from the outside of the reactor, appeared as a "black mirror." It is of special interest that the inlet section of the tubular reactor did not have any coke deposits. This section was the one that was subjected to increasing temperatures in the furnace. The start of the coke deposits occurred approximately in the section where maximum temperatures occurred during a run. Most of the deposits appeared to occur in the

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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exit section of the reactor, which was the portion of the reactor subjected to decreasing temperatures. The location of the coke deposits is consistent with the postulate that heavy condensation products were formed in the gas phase and that these heavy products were deposited or condensed in the cooler sections of the reactor. The smooth finish of the deposits on the inner walls can be explained by the postulate that initial deposits from the gas phase were liquid or semiliquid; the initial deposits then reacted (especially at higher temperatures) to yield the smooth coke deposits plus hydrogen. This proposed mechanism for coke production from butadiene would result in coke that contains little or no metal granules. This conclusion explains why the coke on the wall of the Incoloy 800 reactor in Run 14 resulted in a very inactive surface, whereas the metal-containing coke formed in Runs 15 and 18 was very active. Benzene Results Almost no reactions occurred when a mixture containing about 12% benzene and the remainder helium was passed through Vycor glass and alonized Incoloy 800 reactors in the 550°-700°C range. The concentration of hydrogen in the exit gas stream, which is related to the conversion of benzene, is reported as a function of temperature in Figure 2. Complete conversion of benzene to carbon and hydrogen would result in a hydrogen concentration of about 29%; such a line is shown in Figure 2. At 750° and 800°C, a small amount of benzene (up to about 0.4% ) reacted in the alonized reactor. Hydrogen and coke were the main products. Traces of methane, ethylene, carbon dioxide, and water also were noted. The latter two compounds were apparently the result of trace amounts of metal oxides that had remained after pretreatment of the reactor, first with oxygen and then with hydrogen. Traces of carbon dioxide and water also were detected for runs in the Vycor glass reactor that was also pretreated first with oxygen and then with hydrogen. Significant benzene conversions, however, occurred in the Incoloy 800 reactor at temperatures as low as 500°-550°C, as shown in Figure 2. Hydrogen, coke, and a heavy compound (probably diphenyl) were formed. The reactor had been pretreated prior to the run first with oxygen to remove coke deposits and then with hydrogen to reduce the metal oxides formed from oxygen. The results of the eight samples taken during the run are shown in Figure 2. Initial benzene conversions were high but as the run progressed, the conversions tended to decrease. Comparisons of the results of Samples 2 and 8 and of Samples 4 and 7 clearly show this trend. It also is of interest that a lower conversion occurred for Sample 7 at 700°C compared with Sample 5 at 650°C. Samples 4, 5, and 6 all indicate benzene conversions of 90% or greater.

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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11.

ALBRIGHT AND YU

500

Tubular Pyrolysis Reactors

550

600

650

700

TEMPERATURE

201

750 8 0 0

°C

Figure 2. Hydrogen concentration in product gas stream during benzene pyrolysis in various reactors. 12% benzene feed: (Ώ) Vycor, (Q) Incoloy 800, (·) Alon. 9

Methane was formed in significant amounts in such cases; it is thought that hydrogen reacts with the surface coke or metal carbides on the surface, such as has been shown earlier to occur (J). At the same tem­ peratures ( 6 0 0 ° - 7 0 0 ° C ) , little or no reactions occurred in the Vycor glass or in the alonized Incoloy 800 reactors. Clearly the Incoloy 800 surfaces were promoting significant coking reactions at these temperatures. Based on the results of this run, the coke formed on the Incoloy 800 surface is quite inactive. Probably even lower conversions would have occurred if the run had continued longer and if more coke had been allowed to form on the surface. Unfortunately, no attempt was made to inspect or analyze the coke formed from benzene. It would be of special interest to determine how much metal was incorporated in the coke formed. Additional Testing of Alonized Incoloy 800 Reactor A series of runs was made in the alonized Incoloy 800 reactor by using ethylene, ethane, propylene, and propane. These runs were com-

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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parable with those of Brown and Albright (2) who had used Incoloy 800, stainless steel 304, and Vycor glass reactors. The conversion results of the runs using the alonized Incoloy 800 reactor were quite similar in all cases to the results obtained in this investigation for the same hydrocarbons in a reduced stainless steel 304 reactor and also to those results obtained earlier ( 2 ) in another reduced stainless steel reactor. Several tests were made to compare the results obtained in the alonized reactor after pretreatment with oxygen, after pretreatment with hydrogen, and after no pretreatment. These runs, made with ethylene and propylene, indicated that pretreatments had little or no effect on the pyrolysis conversions. Tests reported earlier (2) with Incoloy 800 and stainless steel 304 reactors indicate that similar pretreatments had important effects on the conversion levels. In these latter two reactors, it is obvious that significant amounts of metal oxides are produced on the inner surfaces during oxygen pretreatments. Such surface oxides subsequently react with hydrocarbons to form both carbon oxides and water. In the case of the alonized Incoloy 800, apparently little or no metal oxides (except for alumina) were formed or destroyed as a result of oxygen and hydrogen pretreatments. It is probable that surface roughening occurs to at least some extent as a result of surface oxidation and then subsequent reduction of high-alloy steel reactors. Since alonizing minimizes this sequence of surface oxidation and reduction, surface roughening probably occurs to at least a reduced extent in alonized tubes; the results with the scanning electron microscope of Albright, McConnell, and Welther (3) support this conclusion.

Discussion of Results The findings of this investigation help explain certain coking phenomena noted by Dunkleman and Albright (10) and also by Albright and McConnell (11). They had found, for ethane pyrolyses in laboratory tubular reactors, that relatively low rates of coke formation sometimes resulted after a short period of operation of a just-cleaned, Incoloy 800 tube but that high and essentially uncontrolled levels of coke formation occurred in other cases. The results of the present investigation suggest the cause of these two phenomena. In the former case, coke that deactivates the surface is probably initially formed and deposited on the surface. Such a coke is presumably similar to that formed from butadiene in the present investigation. This coke would likely have little or no metal granules incorporated in it. In the latter case, the initial coke formed in the just-cleaned reactor probably contains a large amount of metal granules that further catalyze coke formation. Coke such as was

Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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formed in Run 15 and especially in Run 18 is an example of the latter (and undesired) coke. It would be preferred that during the start-up phases of a new or just-cleaned reactor an inactive coke be produced. The results of this investigation and particularly of those with buta­ diene strongly suggest that at least portions of the inactive coke formed during pyrolyses involve the following sequence of events: (a) produc­ tion in the gas phase of unsaturated hydrocarbons, ( b ) chemical conden­ sation or polymerization of unsaturated hydrocarbons to produce rather heavy hydrocarbons, (c) physical condensation of these heavy hydro­ carbons as liquids on the reactor walls or in the transfer line exchangers, and (d) decomposition of the liquids to coke (or tars) and hydrogen. This sequence of events is essentially identical to the one proposed by Lahaye et al. (12) for coke production from cyclohexane, toluene, or n-hexane. Although more information is needed to determine details concerning factors that favor inactive coke formation, relatively high levels of surface sulfides probably promote formation of such coke. On the other hand, metal oxides on the surface likely favor production of active coke. Sulfid­ ing the reactor tube immediately upon completion of the decoking step would form metal sulfides. An aluminized surface, such as provided by the alonized Incoloy 800 reactor, also has been found to be an effective way to prevent the production of active coke. Quite possibly, the initial type of coke formed on the just-cleaned tube would have an important effect on the length of time a reactor tube could be used in a commercial plant before decoking would be required.

Literature Cited 1. Tsai, C. H.;

Albright,

L. F.

"Industrial

Symp. Ser. 1976, 32, chapter 16.

2. Brown,

S. M.; Albright,

L. F.

"Industrial

Symp. Ser. 1976, 32, chapter 17.

3. 4. 5. 6.

and Laboratory

Pyrolyses,"

ACS

and Laboratory

Pyrolyses,"

ACS

Albright, L. F.; McConnell, C. F.; Welther, K. Chapter 10 in this book. Lobo, L. S. Trimm, D. L. J. Catal. 1973, 29, 15. Bernardo, C. Α.; Lobo, L. S. J. Catal. 1975, 37, 267. Baker, R. T. K.; Harris, P. S.; Thomas, R. B.; Waite, R. J. J. Catal. 1975, 30, 86. 7. Baker, R. T. K.; Waite, R. J. J. Catal. 1975, 37, 101. 8. Baker, R. T. K. Chem. Eng. Progr. 1977, 73(4), 97. 9. Sakai, T.; Soma, K.; Sasaki, Y.; Tominga, H.; Kunugi, T. In "Refining ;

Petroleum for Chemicals," Adv. Chem. Ser. 1970, 97, 68. 10. Dunkleman,

J. J.; Albright,

L. F.

In "Industrial

and Laboratory

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Oblad et al.; Thermal Hydrocarbon Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

Pyrolyses,"